1. Technical Field
The present invention generally relates to an oscillator, and more particularly, to a wide bandwidth voltage controlled oscillator using a negative resistance technique.
2. Description of Related Art
In recent years, due to the development of the wireless communication, the manufacturing technology of integrated circuit (IC) has a violent variation within a short time, especially for the circuit of the transmitter applied in radio frequency (RF). With rapid progress in semiconductor manufacturing technology, the current application of IC at Giga Hertz becomes wider and wider, and even progresses to higher operation frequency. Accordingly, the design of the circuit of the transmitter is more difficult and has more challenges. A voltage controlled oscillator (VCO) plays an important role in the front end RF circuit. The VCO is mainly used in the frequency synthesizer to provide a stable and variable reference frequency in RF system.
In IC design, an LC-VCO is designed generally with the configuration of negative resistance, and the frequency of the LC-VCO is changed with a variable capacitor. The oscillation frequency fOSC of the LC configuration thereof is determined according to the equation fOSC=1/(2π√{square root over (LC)}). From this equation, only the inductance L and the capacitance C can be changed to tune the oscillation frequency fOSC. FIG. 1 illustrates a conventional VCO 100 using a negative resistance circuit (NRC). Transistors Mn1 and Mn2 form the NRC. In FIG. 1, CV represents a variable capacitor, C0 represents an invariable capacitor, and CP represents a parasitic capacitor. By using the mutual resonance of the inductor and the capacitor, the inductor and the varactor can serve as the oscillation frequency of the circuit. The oscillation frequency fOSC thereof is determined according to the equation
      f    OSC    =            1              2        ⁢        π        ⁢                              L            ×                          (                                                C                  0                                +                                  C                  P                                +                                  C                  V                                            )                                            .  
Conventionally, the chip inductor adopted in the high frequency IC is a helical inductor realized in metal layers. Because the inductance of the inductor L is hard to be changed, the conventional VCO 100 includes a capacitance device related to voltage characteristic—the variable capacitor CV. By adjusting the variable capacitor CV, the oscillation frequency fOSC is changed, and all kinds of required frequency specifications are synthesized. Besides the variable capacitor CV, the equivalent capacitor of the conventional VCO 100 includes the practical parasitic capacitor CP and the invariable capacitor C0. Accordingly, the entire tuning range is determined by the proportion of the capacitance of the variable capacitor CV relative to all of the equivalent capacitance in the VCO 100. Unfortunately, for some capacitance, if the VCO is desired to operate at high frequency, the equivalent resonance capacitor requires being relatively reduced. Because the practical parasitic capacitor CP is generally inevitable, and it is required to reduce the total equivalent capacitance, the proportion of the capacitance of the variable capacitor CV relative to all of the equivalent capacitance is reduced. Accordingly, the variable range (i.e. the tuning range) of the oscillation frequency fOSC is limited. In other words, in order to maximize the tuning range, the variable range of the capacitance of the variable capacitor CV requires being increase. However, it may reduce the oscillation frequency fOSC because the total equivalent capacitance become larger. Therefore, these issues are traded off between the oscillation frequency fOSC and the tuning range. In the design of high frequency circuit, it is affected considerably.